Infrared detectors, (such as Dewar detector assemblies) contained within a sealed cryo-vacuum assembly have been known for a number of years. In general, these types of infrared detector assemblies have a modular metal and glass construction and are vacuum sealed. A cold cavity assembly penetrates the vacuum housing in order to extract heat generated from the FPA to maintain a specific temperature. The cold cavity assembly typically provides a transparent glass window at one end and, opposite from the window and exposed to the internal vacuum, an infrared focal plane.
The focal plane is typically an array of tiny pixels, each pixel being operable to generate or pass a current in response to infrared radiation being incident upon the pixel. In most cases the generated or passed current is proportional to the incident infrared energy.
As both small and highly sensitive devices, these pixels are prone to some fluctuation in their response behavior over the life of the infrared detector. Regular calibration is typically highly desired so as to insure uniformity in measurements. It is important to know the relative operation performance of the pixels relative to themselves. An unknown change in performance could lead to erroneous data regarding measurements of an infrared source. As such, calibration is used to correct for pixel to pixel variations, such as a non-uniformity correction, and changes in the pixel response over time.
Typically, the calibration is accomplished by placing a large source at or near the infrared detector aperture. The source injects illumination into the assembly and ideally illuminates the entire focal plane at the same time. Although calibration in a laboratory environment is typically performed before deployment, regular re-calibration after deployment is highly desired for the reasons stated above.
The calibration flood source is therefore typically provided by a mechanical method of switching between looking at the calibration source or the outside world. Frequently this is accomplished by moving a flood source over the entire aperture, which being a large aperture in some detectors requires a large mechanical system and a large illumination source.
Uniformity of the illumination is very important, and with larger light sources the issue of uniformity become increasingly challenging. To reduce the size of the illumination source and avoid the cumbersome external mechanics to move the source over the aperture, alternative configurations have been attempted. One such alternative is to place the illumination source within the cold cavity assembly and close to the focal plane.
At least two issues quickly arise when the illumination source is placed within the cold cavity assembly. First, if the illumination source is placed in front of the focal plane, then it will also shadow at least part of the focal plane. To avoid such a shadowing affect, at least some component of the illumination source must be actuated so as to move out of the way. Second, and perhaps more critical, the presence of the illumination source within the cold cavity presents an additional thermal load to the detector assembly. This additional thermal load provided by the illumination source will likely degrade performance, retard the response time of performance, and require more cooling power.
As infrared detector assemblies of this type are frequently space based devices, a complex calibration system is quite undesirable. Size and weight are ever present factors in the cost of assembly and launch. The greater one component is in terms of size and weight, generally the smaller some other component must be.
Further, the system must also be designed to withstand the forces incurred during launch and deployment and then repeated operation requests without being easily serviceable. There are also a number of existing elements that are highly desired for detector functionality, such as spectral filter wheels and guidance systems. Options to provide calibration without compromising existing systems has proven challenging as well.
Hence, there is a need for a focal plane calibration system that overcomes one or more of the issues and problems identified above.